Innate immunity via Toll Like Receptors (TLRs) is the first line of defense against microbial invasion and is essential for pathogen detection. Our preliminary data, using experimental murine systems, also demonstrates that TLR dependent immunity is important for alloimmune responses by allowing the maximal function of dendritic cells (DCs), specifically the production of proinflammatory cytokines that subsequently initiate TH1 alloimmunity. Although these effects are critical for the rejection of minor mismatched skin allografts they are not essential for the rejection of fully allogeneic skin and cardiac allografts. However, the role of innate immunity in transplantation tolerance remains unexplored. Since a prior report, using non-transplant in vitro models, demonstrated that T regulatory cell (T reg) function changes in the presence of TLR stimulated DCs, our preliminary data investigate the impact of innate immunity on transplantation tolerance and show that absence of MyD88, an important TLR signal adaptor, is critical for tolerance induction. We provide evidence that this occurs via a regulatory mechanism that is associated with increased numbers of CD4+CD25+ T cells and a reduced proinflammatory milieu. Therefore, this proposal will employ a murine experimental transplant model and will examine the mechanisms by which defective innate immunity facilitates transplantation tolerance. Aim 1 of this proposal will examine whether the absence of MyD88 operates in synergy with the tolerance protocol (costimulatory blockade), tipping the balance towards tolerance rather than immunity, by increasing the generation and function of T regs. Aim 2 will investigate whether MyD88 deficiency, by reducing the proinflammatory cytokine environment, increases T effector susceptibility to regulation leading to tolerance induction. Therefore, this proposal will critically address whether an absence of innate MyD88 signaling promotes transplantation tolerance, a previously unexplored concept in the field of transplantation. The information generated will support a new paradigm in the field of transplantation and has the potential to provide future therapeutics translatable to human allograft transplantation and autoimmunity (e.g., MyD88 blockade at the time of tolerance induction). [unreadable] [unreadable]